Metavolcanic and metasedimentary rocks

Unit symbol: Trms
Age range Late Triassic (235 to 208.5 Ma)
Lithology: Metamorphic
Group name: Skolai and Mankomen Groups, undivided
Consists, in part, of the informal Amphitheatre group of Smith (1981), which ranges in western exposures from thinly stratified greenish volcaniclastic metasedimentary rocks, chert, and greenschist interbedded with basalt and dark carbonaceous argillite to thicker bodies of undifferentiated pelitic and volcaniclastic metasedimentary rocks. The thinly stratified volcaniclastic sedimentary rocks have common graded laminae, cross bedding, and other depositional textures. The thicker undifferentiated pelitic and volcaniclastic metasedimentary rocks typically show lateral change from argillite to thin tuffaceous interbeds (Smith, 1981; Smith and others, 1988). Unit also includes clayey and silty lenses and interbeds of microsparite that are locally carbonaceous, laminated, or fossiliferous; presence of Halobia cf. H. superba and Tropites sp. indicate a Late Triassic age. Marble contains structures suggestive of scleractinian colonial corals such as Thamnastraea(?). Smith (1981) also reported thickly bedded greenish flows that show numerous exposures of columnar jointing and pillows, and he suggested that these and their underlying rocks may be as old as late Paleozoic. Eastern exposures, such as in the Mount Hayes and Nabesna quadrangle, are chiefly fine-grained quartz–white-mica schist, chlorite-calcite-quartz schist, marble, greenstone, and sparse metadacite that exhibits a weak schistosity that strikes northeast to east and dips moderately to steeply northward (Nokleberg and others, 1992a). This map unit is contiguous with rocks of the Permian to Pennsylvanian Mankomen and Skolai Groups (unit PIPms and subunits), but available data does not allow distinction at map scale)

Source map information

Source map Nokleberg, W.J., Aleinikoff, J.N., Lange, I.M., Silva, S.R., Miyaoka, R.T., Schwab, C.E., and Zehner, R.E., with contribution for selected areas from Bond, G.C., Richter, D.H., Smith, T.E., and Stout, J.H., 1992, Preliminary geologic map of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Open File Report 92-594, scale 1:250,000 [Received as CAD coverage produced in 1996].
Symbol csv
Unit name Schistose metasedimentary and metavolcanic rocks.
Description Chiefly fine-grained quartz-white mica-schist, chlorite-calcite-quartz schist, marble, greenstone, and sparse metadacite. Protoliths of quartz-siltstone, calcareous silty sandstone, limestone, fine-grained basalt, and fine-grained dacite. Unit exhibits a weak schistosity striking northeast to east and dipping moderately to steeply northward.
Lithology Metamorphic

Correlated geologic units

Label Trnm
Description Schistose metasedimentary and metavolcanic rocks, Late Triassic
Geologic age Carnian to Norian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Quartz-feldspar-schist < Schist < Metamorphic Greenschist Major
Greenstone < Metaigneous < Metamorphic Greenschist Indeterminate, major
Metabasalt < Metavolcanic < Metaigneous < Metamorphic Greenschist Indeterminate, major
Metadacite < Metavolcanic < Metaigneous < Metamorphic Greenschist Indeterminate, major
Marble < Metacarbonate < Metasedimentary < Metamorphic Greenschist Indeterminate, major